1. Field of the Invention
This invention relates to a fluorescent display device, and more particularly to such a type of fluorescent display device that luminous display can be observed through an external surface of a transparent substrate, the internal surface of which anode sections are formed.
2. Description of the Prior Arts
A fluorescent display device performs luminous display in the form of letters, figures and the like by selectively impinging electrons emitted from cathodes on anodes having phosphor layers deposited thereon. Such a fluorescent display device has several advantages when it is used. For instance, it is easy to observe its luminous color, and also it can be driven at a low voltage and is low in power consumption. Accordingly, the fluorescent display device has been extensively used in display portions of various electronic devices and instruments.
Conventional fluorescent display devices are generally divided into two types, namely, the type of observing luminous display of phosphor layers deposited on the upper surface of anode sections formed on an insulating substrate through grids, cathodes and a front glass and the type of observing the luminous display of the phosphor layers through a transparent substrate on which anode sections made of a transparent conductive film and coated with phosphor layers are formed.
In the conventional fluorescent display devices of the former type, electrode sections within the display tube are visible from the outside and the shoulder portions of a front glass cover obstruct the observation of luminous display, which results in reducing the visual field of luminous display. Such a disadvantage is further promoted by grids, because the grid holders obstruct the observation of luminous display and restrict the visual field. In the conventional fluorescent display device having a tetrode structure for effecting multi-digit dot-matrix display which includes a grid for selecting a row or column of dots and a grid for selecting a digit to be displayed, luminous permeability from phosphor layers is variable depending on viewing angle, because both the grids visually overlap each other. In the conventional fluorescent display devices using a filter for making grids and other elements invisible, the luminous brightness is significantly decreased.
The fluorescent display devices of the latter type is disadvantageous in that the contrast between the luminous and non-luminous portions is unsatisfactory and the resulting display is hard to observe because light emitted from phosphor layers in the direction opposite to the viewing direction of the display portion is reflected by grids, the inner surface of a front glass or the like disposed opposite to the phosphor layers and is dispersed through transparent electrodes and substrate.